1. Field of the Invention
This invention relates broadly to eye surgery. More particularly, this invention relates to refractive laser systems for eye surgery.
2. State of the Art
The laser refractive surgery (or laser keratectomy) field has rapidly grown over the past few years with many new lasers and algorithms to correct human vision. Systems are now using laser wavelengths from the ultraviolet (excimer) to the infrared to change the shape of the cornea in a calculated pattern which makes it possible for the eye to focus properly. For example, in the treatment of myopia, the excimer laser is used to remove or ablate tissue from the cornea in order to flatten its shape. Infrared (IR) energy is also used by some companies to treat myopia by reshaping the corneal tissue by a “thermal” method as opposed to ablation with the excimer wavelength. The correction of hyperopia is produced by steepening the cornea by removing tissue at the outer edges of the cornea (excimer) or by reshaping the cornea at the outer edges (IR energy). The correction of astigmatism, both myopic and hyperopic, requires the laser to remove or reshape tissue in a more complex pattern.
Initial systems approved by the FDA implement the refractive corrections by a broadbeam approach; i.e., by delivering beam-shaped laser energy based on thin lens theory and paraxial optics applied to a single spherical surface. The beam is shaped by a motorized iris (myopia and hyperopia) and a motorized slit (astigmatism) configured according to profiles derived through Munnerlyn's derivation (C. R. Munnerlyn, S. J. Koons, and J. Marshall, “Photorefractive keratectomy: a technique for laser refractive surgery”, J. Cataract Refract. Surg. 14, 46-52 (1988)). Referring to FIG. 1, more particularly, an excimer broadbeam laser beam 10, typically having a raw rectangular shape measuring 8-10 mm by 20-25 mm and shaped by optics into a 7-10 mm square or circle, is projected onto a motorized, mechanical iris 12 to create a two dimensional (2-D) circular ablation pattern for treating myopia, and onto a motorized, mechanical slit 14 to create a 2-D rectangular ablation pattern for treating astigmatism, and together forming the combined 2-D pattern 16. Thus, the large rectangular laser beam is shaped to form a circle or a smaller rectangle. These shapes are then projected with an imaging lens 18 onto the cornea 20 of the eye 22 in a controlled manner to perform the refractive correction. To create a refractive correction, a correctly shaped volume of tissue must be removed. Referring to FIG. 2, this volume of tissue is removed by firing a series of laser pulses (1, 2, 3, . . . , n) through the iris and/or slit in a controlled fashion to create a three dimensional (volumetric) etch. This is the most common method in the commercial market today and is currently used by VISX and Summit. Referring to FIG. 3, as a mechanical iris is used to create the “circular” part of the ablation pattern, the etch is not perfectly circular. Physical irises possess a finite number of blades. For example, VISX uses a 12-leaf (12 blades) iris, while Summit used a 14-leaf iris. Therefore, the resolution of the ablation patterns through the iris (FIG. 3) is far from the ideal (FIG. 4). Moreover, the choice of ablation patterns (circular, rectangular, or a combination thereof), is constrained by mechanical limitations.
A more recent approach to laser keratectomy uses a scanning laser spot system in which a small laser spot (typically 0.5 mm to 1.0 mm in diameter) is scanned across the cornea in a predetermined pattern to achieve refractive corrections. These systems differ in that they are more flexible than the broadbeam approach. With the control of a small spot, different areas of the cornea can be shaped independently of other areas. The scanning spot system has the added advantage of being able to ablate smaller regions of the cornea (0.5 to 1.0 mm spot size) so it can be directed to ablate more complex, customized patterns (as opposed to the broadbeam approach).
Recently, corneal topography maps have been used to reveal that the cornea has many minute variations across the cornea. The broadbeam laser approach ablates an equal amount of tissue from the high points and low points of the corneal surface so that the original contour of the surface remains (compare FIGS. 5a and 5b which show exaggerated variations of a greatly enlarged minute location). The broadbeam laser cannot correct these minute variations. Initial scanning spot systems also failed to accommodate surface contours. Yet, the introduction of the scanning spot laser has allowed more controlled treatment and thus corneal topography-driven treatments have been produced. For this procedure, the surface topography of the eye is considered along with the refraction correction profile. Thus, Munnerlyn's equation, or any other higher order model, is combined with the eye surface topography to achieve a better refractive correction. To derive the ablation profile, the corneal profile (topographical data), as shown in FIG. 6a, is first determined from the corneal topography system measurements. Next, the topographical data is compared to the ideal corneal shape, e.g., a sphere or asphere, without correction. Referring then to FIG. 6b, the difference between these two is determined at each x,y point in the cornea topographical data array (a digitized image). Then, a profile which eliminates the topographical data (hills and valleys) is generated leaving an ideal surface after which the refraction correction ablation profile is applied. Alternatively, the topographical differences can be combined into the refraction correction ablation profile and the entire combined profile can be applied all at once. For either method, the scanning spot approach allows treatment in isolated areas (versus broadbeam), and thus a pattern of spots is applied to attempt to correctly match the topography.
The corneal topography approach compensates only for topographical aberrations at the corneal surface. However, the eye is a complex optical system of which the cornea is only one component. Thus, the current refraction correction equation, as derived by Munnerlyn, is not capable of suggesting what correction must be made to the corneal shape in order to optimally correct for the overall aberration of the eye's optical system.
There have been several recent approaches to the above problems. First, by expanding the mathematical equations for refraction correction to include higher order effects, coma (3rd order) and spherical (4th order) aberrations can be reduced. See C. E. Martinez, R. A. Applegate, H. C. Howland, S. D. Klyce, M. B. McDonald, and J. P. Medina, “Changes in corneal aberration structure after photorefractive keratectomy,” Invest. Ophthalmol. Visual Sci. Suppl. 37, 933 (1996). Second, by improving schematic model eyes to include higher order aberrations, these new models can provide insight into how the various elements of the eye optical system correlate to affect visual performance. For example, there is a general consensus that the negative asphericity of the normal cornea contributes a negative aberration content. The negative aberration is compensated for by a positive aberration contribution from the gradient index nature of the lens. See H. Liou and N. A. Brennan, “Anatomically accurate, finite model eye for optical modeling,” J. Opt. Soc. Am. A, Vol. 14, 1684-1695 (1997). The convergence of work on modeling the human optical system with more accurate mathematical descriptions for refraction correction has led to the development of advanced ablation profile algorithms that treat the cornea as the first aspheric element in an optical system. See J. Schwiegerling and R. W. Snyder, “Custom photorefractive keratectomy ablations for the correction of spherical and cylindrical refractive error and higher-order aberration,” J. Opt. Soc. Am. A, Vol. 15, No. 9, 2572-2579 (1998).
Therefore, more recently, a number of wavefront sensing systems are being developed for the scanning spot laser refractive market. In these systems, a visible laser beam, or a number of visible laser beams, is/are directed through the entire eye optical system: cornea, lens, vitreous and retina. The return reflection from the retina is recorded by a CCD camera and analyzed against an ideal wavefront. Thus, the entire optical system is analyzed. The result of this analysis yields a simulation of best acuity for the patient. This data can be used to make an exact contour ablation of the cornea. Regardless of which technique is used, the result is a contour topographical map yielding height information from the current corneal shape to the shape calculated to best improve visual acuity with a scanning spot.
However, there are several problematic issues with all scanning spot systems. First is the issue of treatment time. Scanning spot systems require longer refractive surgery times. The scanning spot is a slower approach since the small laser spot has to be moved over a wide surface (up to 10 mm for hyperopia). The scanning spot system typically delivers several hundred spots per treatment layer, and consequently treatment times are relatively long. The broadbeam approach is much quicker as the entire cornea is treated with each laser pulse, or treatment layer.
Second is the issue of safety. The broadbeam laser is inherently safe from a treatment interruption standpoint because the cornea is treated symmetrically for each pulse; the iris represents a circle and the slit represents a rectangle so that every point on the cornea being treated is treated the same with each laser pulse. If the procedure is interrupted, there will always be some symmetrical spherical correction which can be continued more easily at a later time. However, the scanning spot, with its small spot size, cannot cover the entire corneal surface with one laser pulse. Thus, if an interruption occurs, there is no guarantee of a complete corneal etch for a layer at the point of interruption. Continuation at the point of interruption would be difficult.
Third is the issue of tracking. In the scanning spot system the eye needs to be tracked very carefully in order to deliver the spot to the correct point on the cornea as the eye moves. This is not as much of a problem in the broadbeam system as a broader area of the cornea is treated with each pulse.
Fourth is the issue of surface roughness. Due to the circular cross-sectional shape of the laser spot, the scanning spot technique necessitates overlap of the laser spot as the laser spot is moved in a raster scan (left to right, top to bottom) over the cornea (see FIG. 7). While it is necessary to overlap spots to provide complete coverage for a given ablation zone (a typical 80/20 overlap is shown in FIG. 7), regions of overlap will be ablated at twice the etch depth per pulse. This tends to create roughness in the resulting etch. The roughness of the ablated volume is dependent on the spot overlap and to a lesser extent, the ratio of spot diameter and ablation zone diameter. This problem is not seen in the broadbeam approach.
Fifth is the issue of resolution. The laser spot typically has a diameter of between 0.5 mm and 1 mm. However, corneal topography and wavefront sensor analysis provide detailed information about the required correction to the cornea, and such details may require ablation at a resolution greater than 0.5 mm. For example, referring to FIG. 8a, both corneal topography and wavefront sensor analysis provide images which defines several topography zones requiring ablation. One such topography zone is isolated in FIG. 8b. Yet, referring to FIGS. 8c and 8d, neither a 1 mm spot or a 0.5 mm spot, respectively, are sized to ablate the topography zone at the desired resolution.
In response to the problems associated with the current broadbeam and scanning spot system, U.S. Pat. No. 5,624,437 to Freeman, which is hereby incorporated by reference herein in its entirety, discloses the use of a digital micromirror device (DMD) to redirect a broadbeam laser pulse to the eye. The DMD includes over a million individually configurable mirrors each having a square reflective surface approximately 13 to 16 microns per side. The mirrors are configurable into refractive correction patterns of very high resolution, and the laser energy is reflected by the mirrors into appropriate corrective patterns on the eye. The system has none of the disadvantages associated with prior broadbeam and scanning spot systems, but the advantages of each are provided.
In view of the superior resolution capable with the Freeman DMD laser system, it is desirable to utilize a DMD laser system which is adaptable to perform any type of laser ablative pattern: broadbeam circular and rectangular patterns, scanning spot circular and rectangular patterns, corneal topography patterns, and wavefront sensor analysis patterns.